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Calcium is an essential nutrient for avian reproduction. Breeding birds need calcium for eggshell production, and for the skeletal growth of the nestlings. A number of studies have documented that low level of calcium availability can greatly affect the eggshell thickness, egg size, clutch size and hatching success. Calcium-supplementation has a positive effect not only on the mentioned reproductive parameters but also on nestlings’ female condition. Extra calcium also leads to an start of breeding and reduces frequency of clutch desertion. Hence, calcium availability may limit reproduction in birds.Specific hunger for calcium is one of the most important adjustment aimed at increasing calcium level in the body and it is demonstrated by specific foraging behaviour. Females store calcium in medullary bone which is a unique to birds, labile type of bone from which calcium can be mobilized much faster than from structural bone. There are two basic strategies in calcium management: 1) accumulation of Ca in bone during a long period before reproduction (so-called ‘capital breeders’) or 2) obtaining calcium rich-material up-to-date, during breeding season, without prior storing calcium (‘income breeders’). Among passerines 91% belong to ‘income breeders’. The capacity for using endogenous reserves is quite limited in small passerines laying relatively large clutches hence, many species have to rely on additional calcium-rich material like snail shells, bones, eggshells and calcareous grit during egg-laying and when feeding young. Obtaining sufficient calcium is time- and energy-consuming, even in calcium- rich environments. Hence, the real cost of egg formation of passerines in calcium- poor areas may involve much more than the current estimate of 15% of the daily energy expenditure of the female. It is proved that birds respond to a decrease in the calcium supply by increasing their search effort for calcium. There exist a trade-off between the time spent searching for calcium-rich items and time spent foraging for other nutrients. Searching for this micronutrient takes place also at the expense of resting, which involves higher energy expenditure. Calcium deficiency may cause decline in female body mass and concentration Ca in females bones in passerines.
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Ptaki lęgowe zabudowy mieszkalnej Siedlec

67%
Pb and Cd concentrations in the livers in the relation to the development and mortality of Passer domesticus and P. montanus nestlings in urban parks of Warsaw and suburban villages were studied. Pb concentrations in the livers of both species were higher in sick and dead nestlings than in healthy ones. The proportion of sick and dead nestlings was increased in the classes of Pb concentration higher than 2 µg·g⁻¹ of dry weight. The mean deviations of the body weights of healthy and sick nestlings from the model weight were lowest at Pb concentrations in the liver less than 2 µg·g⁻¹ of dry weight. No significant differences were found in Cd concentrations in the livers of healthy, sick and dead nestlings. Proportions of these categories of nestlings in different classes of Cd concentrations were similar.
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